The invention relates to a mechanism for operating one or more valves of a piston device, in particular an intake valve and/or an exhaust valve of an internal-combustion engine.
A valve-operating mechanism is known from FR 2 480 854. This known mechanism comprises a first plunger and a second plunger, each plunjer being operated by a corresponding cam. These cam operated plungers and the piston for actuating the valve delimit a common chamber which is filled with hydraulic fluid. The common chamber of the known mechanism is provided with a check valve controlled opening. Via this opening hydraulic fluid can be supplied to the common chamber to compensate for play in the valve-operating mechanism.
The invention relates in particular to a mechanism which makes it possible to make opening and closing the valve(s) variable to a considerable extent, in a particular embodiment as a function of the demand for torque in an internal-combustion engine.
The object of the invention, in a particular embodiment, is to provide a valve-operating mechanism which makes it possible to operate an internal-combustion engine using the so-called Miller cycle. At the end of the 1940s, Miller proposed that an internal-combustion engine working on the spark-ignition principle be provided with a system in which, without using a so-called compound cylinder, the compression ratio and expansion ratio are significantly different from one another. Miller achieved this by providing the intake valve of a 4-stroke engine with a very large after-closure.
One drawback of the Miller cycle is that the specific output of the engine falls considerably if the engine has fixed opening times for the intake and exhaust valves. The invention aims to make it possible on the one hand to fully utilise the advantages of the Miller cycle under partial load yet, when full power is required, to completely or partially switch off the Miller cycle.
The present invention provides a mechanism for operating a valve of a piston device, in particular an intake or exhaust valve of an internal-combustion engine, which valve can move between an open position and a closed position and is provided with associated restoring means for returning the valve to the closed position.
The mechanism comprises:
a hydraulic valve actuator for operating the valve of the piston device, which valve actuator has a variable chamber which is delimited by a piston which can be coupled to the valve, in such a manner that when hydraulic fluid is supplied to the said chamber the valve opens,
a rotatable first cam with a first cam profile,
a rotatable second cam with a second cam profile,
the first cam profile and the second cam profile each comprising
a rising flank, a stationary flank and a falling flank,
a first cam follower which can be operated by the first cam,
a second cam follower which can be operated by the second cam,
the relative angular position of the first cam and the second cam with respect to one another being adjustable,
a pressure actuator in which there is a pressure chamber having a variable volume and having a first and a second plunger, the first cam follower being coupled to the first plunger and the second cam follower being coupled to the second plunger, in such a manner that a plunger is stationary if the associated cam follower is operated by the stationary flank, the first and second plungers each being displaceable between a retracted position and an extended position, in such a manner that the position of the first and second plungers defines the volume of the pressure chamber, the retraction of a plunger leading to a reduction in the volume of the pressure chamber,
the pressure chamber of the pressure actuator being connected to the variable chamber of the valve actuator to form a common chamber,
the common chamber being provided with a common chamber opening which common chamber opening has an associated valve assembly for opening and closing the common chamber opening.
The valve assembly associated with the common chamber opening is adapted to open said common chamber opening if the first plunger moves to the extended position or if the second plunger moves to the retracted position, so that hydraulic fluid can flow out of the common chamber.
If the common chamber opening is open the hydraulic pressure in the common chamber is insufficient to open or hold open the valve operated by the hydraulic valve actuator.
The mechanism is designed in such a manner that, at the moment at which the first plunger begins to move out of the extended position into the retracted position, the second plunger is in its retracted position or has almost reached its retracted position, in such a manner that the valve which is operated by the valve actuator opens as a result of the retracting movement of the first plunger.
The adjustability of the angular position of the cams with respect to one another allows virtual cam profiles which differ within a wide range to be obtained. For example, the present mechanism makes it possible, in a 2-stroke internal-combustion engine, to set the valve angle of the intake valve in a continuously variable manner between approximately 100 and 180 degrees while retaining the maximum valve lift. In a 4-stroke engine, the valve angle of the inlet valve can be set between 180 and 360 degrees. In many known mechanisms, there is a relationship between the valve angle and the valve lift, the valve lift generally increasing when the valve angle increases. A relationship between valve angle and valve lift of this type represents a disadvantage.
In the mechanism according to the invention, the first and second cam profiles may be different.
In a piston device with a crankshaft, the angle between the cams may be adjusted, for example, as a result of one of the cams having an angular position which can be adjusted with respect to the crankshaft. It is also possible for both cams to be adjustable independently of one another in terms of their angular position with respect to the crankshaft.
The mechanism according to the invention is particularly advantageous for operating the intake valve of a 4-stroke internal-combustion engine, in particular because the after-closure of the intake valve can be varied easily, so that the traction characteristic of the engine can be optimized. To make this possible, it is advantageous if the angular position of the second cam with respect to the crankshaft of the engine can be adjusted within a considerable range. As a result, it is possible to set a large after-closure at a high engine speed and a smaller after-closure at a low speed. The time at which the intake valve opens can then be varied by also making the angular position of the first cam, variably adjustable within a smaller range.
In 2-stroke internal-combustion engines provided with uniflow scavenging with an exhaust controlled by valves, the mechanism according to the invention also has advantages. Firstly, the pre-exhaust with respect to the point at which the scavenging ports open can now be selected as a function of load and/or speed, since fewer crank degrees are required for pressure equalization at a low speed, but also a lower scavenging factor may be sufficient under a low load, so that the exhaust valves can close earlier, and so that, in 2-stroke engines with pressure charging, the exhaust temperature remains higher, which is advantageous in order to keep the engine operating without an emergency scavenging pump.
In an embodiment which is advantageous in practice, each cam follower is integral with the associated plunger, but in a variant, it is also possible to provide a suitable transmission mechanism between a cam follower and the associated plunger.
The present invention provides for the opening and closing of the valve of the piston device to be completely determined by a hydrostatic process. In this case, the question of whether the opening of the common chamber is open or closed is the determining factor for whether the pressure in the said common chamber can reach a level which is sufficient to open and hold open the valve of the piston device.
Preferably, the valve of the piston device is closed not by means of hydraulic fluid flowing out of the common chamber, but rather entirely on the basis of the movement of the second plunger. As a result, it is possible to ensure that the speed at which the valve moves onto its seat is virtually equal to zero, so that damage to the valve and/or seat and possibly breaking of the valve stem are prevented.
Another possible application is the operation of suction and/or pressure valves of a piston compressor. In this way, it is possible to achieve continuously variable control of the capacity of the compressor from 0-100%. Known compressors can generally only be controlled within 50 to 100% of their capacity, since they are unstable at a lower range of 0-50% of the stroke volume.
A further advantage of the mechanism according to the invention is that it is possible to operate a piston compressor at a higher speed, since the mechanism then also makes it possible to achieve a closing speed of the valves which is equal to zero. Furthermore, the valves lift can be selected to be greater than in known compressors, so that the valve losses are lower. In known compressors, the valve losses may amount to 25% of the isentropically absorbed power. It is also possible for the clearance volume to be small, which is of benefit to the principal dimensions of the compressor.
The invention will be explained in more detail below with reference to the drawing.